Method of preparing concretes of stable aluminous cements

ABSTRACT

A method for preparing stable concrete from aluminous cements is disclosed. This is accomplished by controlling the water/cement ratio between 0.25 and 0.4 when the cement is mixed with water. This ratio insures use of the water for hydration of the binding paste into cubic aluminate.

0 ilited States Patet 51 3,649,318

Stiglitz Mar. 14, 1972 [54] METHOD OF PREPARENG CONCRETES [56]References Cited OF STABLE ALUMINOUS CEMENTS UNITED STATES PATENTS [72]Inventor: Paul Stiglitz, La Violette, France 1,913,943 6/1933 Morgan..l06/l04 [73] Assignee: Societe Anonyme: Ciments Lafarge, Paris,

France Primary Examiner-James E. Feet [22] Filed; J ly 24, 1968Assistant Examiner-W. T. Scott 2 H pp NO: 747,192 Attorney-Brumbaugh,Graves, Donohue & Raymond [57] ABSTRACT [30] yomlgn Apphcauon Pnomy DamA method for preparing stable concrete from aluminous ce- July 31, 1967France ..166375 ments is disclosed. This is accomplished by controllingthe water/cement ratio between 0.25 and 0.4 when the cement is [)2] LS.Cl. mixed with water ratio insures use of the water for hydra- 1m.m-C04b tion of the binding paste into cubic aluminate. [58] FieldofSearch ..106/104, 89

3 Claims, 1 Drawing Figure PATENTEDMAR 14 m2 8,649,318

INVENTOR PAUL STIGLITZ METHOD OF PREPARING CONCRETES OF STABLE ALUMINOUSCEMENTS My invention relates to a method of preparing stable aluminouscements, and further concretes and mortars from said aluminous cements.

It is known that anhydrous aluminous cements consist, for the most part,of calcium aluminates. The other ingredients, silicates,silico-aluminates and calcium ferrites, which are due to impurities inthe raw materials, are considered as having little or no hydrauliceffect at ordinary temperatures.

The hydration products of these cements, causing the setting andhardening of the latter are therefore almost exclusively hydratedcalcium aluminates belonging to the CaO A1 H O system.

This system, which has been extensively studied, can be representedgraphically by a collection of curves, defining, for a giventemperature, the areas in which hydrated aluminates occur, in relationwith the compositions of the solutions, whether stable or metastable, inequilibrium with the solid precipitated phases. The curves themselvesrepresent the compositions of the saturated solutions of hydrates orhydroxides of the system, in water or in lime solutions of variousconcentrations.

There are six solid phases, i.e., three hydrates: monocalcium hydratedaluminates (CaO-A1 O -7l0 H O), dicalcium hydrated aluminate (2CaO-Al O-8H O), tetracalcium hydrated aluminate (4CaO-Al O -l3l-I O); and threehydroxides: tricalcium cubic aluminate (Ca [Al(OH) calcium hydroxideCa(Ol'l) and aluminum hydroxide Al(OH) The only stable phases of thesystem are the hydroxides (Ca(OH Al(O!-l) and Ca [Al(OH)6] All thehexagonal or pseudo hexagonal aluminate hydrates are metastable withrespect to these three phases, due to the fact that they can redissolvein an excess of water to give a supersaturated solution of a stablephase which, at that point precipitates. Their conversion is thereforeunavoidable, although very slow under normal conditions of use ofaluminous cements, and leads, for the latter, to Ca,,[Al(OH) and Al(OH)i.e., to the stable phases of that part of the diagram which is poor inlime.

This conversion, so-called evolution of aluminous cements, is highlyaccelerated in the case when the temperature is increased above 30 C.,which also enhances the conversion of the alumina formed in the firststages of the hydration into stable gibbsite.

At a normal temperature, the first hydration product formed after mixingof the aluminous binder is monocalcium aluminate CaO-Al O -7-10l-l O.Due to the fact that, as previously mentioned, the latter aluminate ismetastable with respect to cubic aluminate and gibbsite, its evolutionis noted, which is usually coupled with a loss in mechanical strength.

Tests carried out by applicant have led to confirmation of the theoryaccording to which conversion leads to the formation of a large quantityof free water which, due to the fact that it remains in the bindingpaste, decreases the strength of the latter. Indeed, the conversion canbe written: 3CaO-Al O -7- 101-1 0 Ca [Al(OH) +4Al(Ol-l) +18H O.

This release of water is further accompanied by an increase in porosity:actually, the volume of three molecules of CHO'AlgOg' lOH- O. while thesum of the volumes of 99 mQle ule of AKOH and four molecules of Al(OH)is only 278.3 emf, i.e., a decrease in solids volume of over 50 percent.

Now, according to FERET, the strength of a hardened cement paste, at a tstage, is given by the formula:

where:

at time of mixing According to Dzulynski, the hydrated productconcentration is the crucial factor, the latter being given by theformula:

where:

'y is the hydrated product concentration Ch stands for the hydratedproduct d, is the free water v stands for the empty spaces (air) theproportions being in volume expressed in the unit of volume of themortar paste.

Compressive strength R is expressed as a function of 'y, by

the exponential formula:

R Roe" where:

R and R are coefficients having a specific value for a given binder,independently from the influence of working characteristics, storage andage.

According to these formulas, it can be seen that a given volume of airhas an effect similar to that of the corresponding volume of water. Theporosity appearing at the time of hexagonal aluminate-cubic aluminateconversion is therefore always harmful to mechanical strength, whateverthe degree of humidity in the mortar paste or concrete may be. Moreover,if water is present, it favors, as mentioned previously, theredissolution of the hexagonal aluminate and the precipitation of thecubic aluminate, i.e., the continuation of conversion up to completion.

On the other hand, the hydration reaction is quite vigorous; the heat ofhydration is given up in a very short time and in order to avoidoverheating of the concrete, it is necessary, on a building yard, tostrip as soon as possible and spray the concrete for at least 24 hoursso as to maintain the temperature below 30 C., which is a constraint forthe users.

In order to avoid these various drawbacks and constraints applicantsidea was to prepare concretes and mortars from aluminous cements undersuch conditions that hydration of the binding paste would occur directlyinto cubic aluminate. To this effect, in order to obtain these concretesand mortars from aluminous cements in accordance with the invention awater/cement ration was used such that the entire mixing water wasemployed for the hydration of the binder into cubic aluminate. Thiswater/cement ratio advantageously ranges from 0.25 to 0.4. All valuesfor water/cement ratios described herein are on a by-weight basis.

When using concretes and mortars prepared according to the invention,stripping may be carried out at any time after hardening has actuallytaken place, and in particular, it is not necessary to cool the concretemass.

Tests carried out on concretes and mortars obtained according to theinvention have shown, under these conditions, that only slightly lowermechanical strengths are obtained as compared with those obtained atnormal temperatures which, instead of decreasing with time, show, on thecontrary a slight increase. These results are summarized in thefollowing table.

While running these tests, the value of (l Rh/Rc) was studied as afunction of the water/cement ratio, where: Rh is the strength of aconcrete containing exclusively hexagonal aluminates, and Re is thestrength of this same concrete comprising only cubic aluminate. Theappended curve shows that, when this water/cement ratio decreases (orwhen the cement/water ratio increases), the value of l-Rh/Rc tendstowards zero, i.e., Rc tends to equal Rh.

The curve shows that equivalency is not reached for the usuallyrecommended water/cement ratio of 0.4 (this equivalency is reached forW/C 0.3 but this small relative difference is quite acceptable when thehigh value of the strength of hexagonal aluminate is considered, thisvalue being measured in the laboratory under working conditions whichmake it possible to obtain this latter aluminate alone, while on thebuilding yard, it is often very difiicult to prevent its entire orpartial conversion. In principle, the strength obtained with cubicaluminate revolves around 400 bars for such a water/cement ratio and,more important, continues to increase instead of decreasing quiteextensively as in the case of hexagonal aluminate conversion. The veryharsh conditions resulting from an 80 C. temperature under which thesevalues were obtained must be taken into account.

0.25 to about 0.4 and thereby insures that the entire mixing water isused for the hydration of the binder into cubic aluminate.

2. A method as claimed in claim 1 in which the water/cement ratio byweight is about 0.33.

3. The product obtained according to the method claimed in claim 1.

22 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 43,31 Dated March '14, 1972 lnvefit Paul 'Stiglitz .It is certified thaterror appears in the above-identified patent and that said LettersPatent are hereby corrected as shown below:

Column 1, line 61, after "10 E 0" insert is 569.6 cm

Column 2, line '16, "R R e should be R R e Y--;

Column 2, line 45, "ration" should read -ratio-;

Column 2, line 67, "Caj [Al(OH) should be Ca [Al(OH) Column 4, line 1,Flow-water" should be -low water- Signed and sealed this 5th day 51September 1972. I

(smL) Atto s t EDWARD I-1.FLETCHER,JR. I ROBERT GOTTSCHALK AttestingOfficer Commissioner of Patents

2. A method as claimed in claim 1 in which the water/cement ratio byweight is about 0.33.
 3. The product obtained according to the methodclaimed in claim